Relationships Between Entrainment and Larval Drift At a Large-River Power Plant Based on 2600 Samples Collected During a 2-Year Period

From April through August 2002 and again during this same period in 2003, larval samples were collected from the Mississippi River and the intake to the Madgett Power Plant to assess possible impacts associated with possibly adding another unit to the plant.  Samples from the river were collected using paired 0.5 m diameter, 500 micron plankton nets from transects upstream and downstream of the intake.  Samples were stratified by depth and position across the river and collected on a diel basis.  In addition to the collections along the transects, a location within the intake’s zone of influence was also sampled.  Seventeen sampling periods were established each year and concurrent with each one, a 24 hr entrainment sample was also collected.  Ultimately, about 2600 samples were collected.  Larval densities peaked both years the second week of June.  The drift was dominated by freshwater drum, gizzard shad, and emerald shiner “types”.  Drum and shad were also common in the entrainment samples.  However, the study showed that a number of other species that were common in adult fish collections we made during this same period were not well represented in the entrainment samples, probably because of life history characteristics of these species.  For example, walleye and smallmouth bass, which are the two most popular sport fish in this area, were common as adults in the river but were rare to uncommon in the drift and the entrainment samples.  In 2002, we estimated that about 19.9 billion eggs and larvae drifted past the plant’s intake but < 1% of these were entrained, a biologically inconsequential number.